# Bézout's identity

Bézout's identity (also called Bézout's lemma) is a theorem in the elementary theory of numbers: let a and b be nonzero integers and let d be their greatest common divisor. Then there exist integers x and y such that

$ax+by=d$

• the greatest common divisor d is the smallest positive integer that can be written as ax + by
• every integer of the form ax + by is a multiple of the greatest common divisor d.

The integers x and y are called Bézout coefficients for (a, b); they are not unique. A pair of Bézout coefficients can be computed by the extended Euclidean algorithm. If both a and b are nonzero, the extended Euclidean algorithm produces one of the two pairs such that $|x|<\left |\frac{b}{d}\right |$ and $|y|<\left |\frac{a}{d}\right |.$

Bézout's lemma is true in any principal ideal domain, but there are integral domains in which it is not true.

## Structure of solutions

When one pair of Bézout coefficients (x, y) has been computed (e.g., using extended Euclidean algorithm), all pairs can be represented in the form

$\left(x+k\frac{b}{\gcd(a,b)},\ y-k\frac{a}{\gcd(a,b)}\right),$

where k is an arbitrary integer and the fractions simplify to integers.

Among these pairs of Bézout coefficients, exactly two of them satisfy

$|x| < \left |\frac{b}{\gcd(a,b)}\right |\quad \text{and}\quad |y| < \left |\frac{a}{\gcd(a,b)}\right |.$

This relies on a property of Euclidean division: given two integers c and d, if d does not divide c, there is exactly one pair (q,r) such that c = dq + r and 0 < r < |d|, and another one such that c = dq + r and 0 < -r < |d|.

The Extended Euclidean algorithm always produces one of these two minimal pairs.

### Example

Let a = 12 and b = 42, gcd (12, 42) = 6. Then we have the following Bézout's identities, with the Bézout coefficients written in red for the minimal pairs and in blue for the other ones.

\begin{align} \vdots \\ 12 &\times \color{blue}{-10} & + \;\; 42 &\times \color{blue}{3} &= 6 \\ 12 &\times \color{red}{-3} & + \;\;42 &\times \color{red}{1} &= 6 \\ 12 &\times \color{red}{4} & + \;\;42 &\times\color{red}{-1} &= 6 \\ 12 &\times \color{blue}{11} & + \;\;42 &\times \color{blue}{-3} &= 6 \\ 12 &\times \color{blue}{18} & + \;\;42 &\times \color{blue}{-5} &= 6 \\ \vdots \end{align}

## Proof

Bézout's lemma is a consequence of the Euclidean division defining property, namely that the division by a nonzero integer b has a remainder strictly less than |b|. The proof that follows may be adapted for any Euclidean domain, and even for any principal ideal domain. For given nonzero integers a and b there is a nonzero integer d = as + bt of minimal absolute value among all those of the form ax + by with x and y integers; one can assume d > 0 by changing the signs of both s and t if necessary. Now the remainder of dividing either a or b by d is also of the form ax + by since it is obtained by subtracting a multiple of d = as + bt from a or b, and on the other hand it has to be strictly smaller in absolute value than d. This leaves 0 as only possibility for such a remainder, so d divides a and b exactly. If c is another common divisor of a and b, then c also divides as + bt = d. Since c divides d but is not equal to it, it must be less than d. This means that d is the greatest common divisor of a and b; this completes the proof.

This proof has the drawback to not provide a method for computing Bézout's coefficients. Bézout's lemma is also a corollary of the proof of the Extended Euclidean algorithm, and this algorithm is an efficient method to compute the Bézout's coefficients. This may also be extended to any Euclidean domain.

## Generalizations

### For three or more integers

Bézout's identity can be extended to more than two integers: if

$\gcd(a_1, a_2, \ldots, a_n) = d$

then there are integers $x_1, x_2, \ldots, x_n$

such that

$d = a_1 x_1 + a_2 x_2 + \cdots + a_n x_n,$

has the following properties:

1. d is smallest positive integer of this form
2. every number of this form is a multiple of d
3. d is a greatest common divisor of a1, ..., an, meaning that every common divisor of a1, ..., an divides d

### For polynomials

Bézout's identity works for univariate polynomials over a field exactly in the same ways as for integers. In particular the Bézout's coefficients and the greatest common divisor may be computed with the Extended Euclidean algorithm.

As the common roots of two polynomials are the roots of their greatest common divisor, Bézout's identity and fundamental theorem of algebra imply the following result:

For univariate polynomials f and g with coefficients in a field, there exist polynomials a and b such that af + bg = 1 if and only if f and g have no common root in any algebraically closed field (commonly the field of complex numbers).

The generalization of this result to any number of polynomials and indeterminates is Hilbert's Nullstellensatz.

### For principal ideal domains

As noted in the introduction, Bézout's identity works not only in the ring of integers, but also in any other principal ideal domain (PID). That is, if R is a PID, and a and b are elements of R, and d is a greatest common divisor of a and b, then there are elements x and y in R such that ax + by = d. The reason: the ideal Ra+Rb is principal and indeed is equal to Rd.

An integral domain in which Bézout's identity holds is called a Bézout domain.

## History

French mathematician Étienne Bézout (1730–1783) proved this identity for polynomials.[1] However, this statement for integers can be found already in the work of another French mathematician, Claude Gaspard Bachet de Méziriac (1581–1638).[2][3][4]